The present invention relates to dispersive delay lines using propagation of surface acoustic waves which are reflected on slanted gratings. These devices are known in Anglo-American literature under the name of "Slanted reflective array compressor", to which the shortened name of "SRAC" has been given, which will be used hereafter.
More particularly, the invention applies to devices with a relatively high band, more especially greater than 50%, and with a high central frequency, for example greater than 1000 MHz. It also applies to the more economical construction of dispersive filters with lower central frequency.
These SRAC devices are known more especially from an article by B. R. POTTER and C. S. HARTMANN in IEEE Transactions on Sonics and Ultrasonics-vol 26, November 1979, pp 411 to 418.
Such a device comprises input and output transducers each having series of interdigital fingers at variable intervals. The transducers and the two reflective gratings are disposed so that to each frequency of the band of the signal there corresponds an input-output path with reflection on the gratings, corresponding to the given dispersion law.
For such transducer devices, the smallest electrode width is equal to ##EQU1## if B is the band, f.sub.o the central frequency and v the speed of the waves. The values of f.sub.o and B are therefore limited by the feasibility of the transducers.
In fact, with even the most sophisticated conventional photolithographic techniques, it is difficult to go below 0.6 .mu.m.
For example, for a frequency band of 1 GHz around 1.5 GHz, the smallest electrode width is equal to about 0.44 .mu.m for a lithium niobate substrate and a propagation along the "Z" axis of the crystal. It is not possible to obtain lines of this width with conventional photolithography.
Moreover, it is known that some types of interdigital transducers are capable of being used at a harmonic frequency of the given frequency by the spacing apart and the width of the electrodes, which allows a higher central frequency to be obtained. This is more especially the case for multi-electrode type transducers. The geometry of these transducers is such that each electrode is divided into several electrodes of smaller width. With these multi-electrode type transducers the undesirable effects due to the reflections of a mechanical nature between the electrodes can generally be eliminated.
In addition, it is known that their frequency response allows them to be operated at different harmonic frequencies. In particular, in the case of a pair electrode transducer, the harmonic lines are of uneven rank and the third has substantially a level equal to that of the fundamental line. Such transducers are described in an article by T. W. BRISTOL et al., appearing in Ultrasonics Symposium Proceedings 1972, pp 343 to 345.
Furthermore, other types of transducers other than the dual electrode type, are capable of operating at different harmonic frequencies.
The disadvantage of transducers operating on harmonics, when the passband of the signal is large, is that the same frequency, corresponding to two different harmonics, may be transmitted by two different fingers.
The dispersive line of the invention remedies these disadvantages and allows high frequencies of the order of 1 GHz and a bandwidth greater than 50% to be obtained at one and the same time, and this with electrode widths obtainable by photolithographic procedures.